Similar to Grouped Data you can derive info from a date, which is very useful in a PluginPivotTable. It is similar to this but directly in Tiki. And it's fast because it's done by PHP.
{display name="tracker_field_TimeSheetDate" default="" format="date" dateFormat="%m"}
{display name="tracker_field_TimeSheetDate" default="" format="date" dateFormat="%W"}
All the options are here:
https://www.php.net/manual/en/function.date.php